The data from about 2600 pairs of parabiosed rats irradiated with 1000 R are now being compiled with regard to incidence of leukemia and incidence of lymphopoietic tumors. Observation of the rats irradiated at dose levels in the 50 to 400 R range is being continued. Data are being compiled from a series of mice irradiated in similar dose ranges. BIBLIOGRAPHIC REFERENCES: Warren, S. and Gates, O.: Leukaemogenesis by Low-Level Radiation. In Biological and Environmental Effects of Low-Level Radiation, Vol. I, IAEA-SM-202/218, I.A.E.A., Vienna, 1976, pp. 169-180.